Fas and the TRAIL receptors, DR4 and DR5, are apoptosis-inducing members of the Tumor Necrosis Factor Receptor (TNFR) family that play a key role in the regulation of immune response. In addition to their known role in the mediation of apoptosis, these receptors are also known to activate the NF-kappaB and JNK pathways. However, the signaling events involved in the activation of these pathways by the Fas and TRAIL receptors have not been characterized. We have discovered that caspase 8 and its homologs, caspase 10 and MRIT, are likely to play a major role in NF-kappaB and JNK activation via Fas and DR4/DR5. These properties of caspase 8 may also explain the recently discovered role of the FADD-caspase 8 pathway in T cell activation and proliferation. The overall objective of this proposal is to further characterize the role of caspase 8 and its homologs in NF-kappaB and JNK activation by Fas and DR4/DR5 and study its biological significance. This objective will be achieved through the following specific aims. In specific aim 1, cell lines deficient in caspase 8, or expressing its mutant constructs, will be used to confirm its role in NF-kappaB and JNK activation by Fas and DR4/DR5. In specific aim 2, the molecular interactions underlying the NF-kappaB and JNK activating abilities of caspase 8 and homologs will be characterized and any novel protein(s) involved in these processes identified. In specific aim 3, role of caspase 8- and its homologs-mediated NF-kappaB and JNK activation in the process of T cell activation and development will be studied by in vitro studies in cell lines and in vivo studies in transgenic animals. We hope that the above studies will not only clarify the role of caspase 8 and its homologs in NF-kappaB and JNK activation by the Fas and TRAIL receptors but also enhance our understanding of the molecular interactions involved in immune regulation.